1. Field of the Invention
The present invention relates to an ink jet head which emits ink liquid droplet from a discharge opening of an orifice plate, perpendicularly to the surface of a substrate bearing a head generating resistance thereon, and a producing method therefor.
2. Related Background Art
The ink jet recording systems have become rapidly popular in recent years, because of its advantages that the noise generation at the recording operation is negligibly small, that the high-speed recording is possible and that the recording can be made on socalled plain paper without any particular processing.
Among the ink jet recording heads, the one that discharges the ink droplet perpendicularly to the substrate bearing the element for generating the ink discharging energy is called the recording head of side shooter type, and the present invention relates to the configuration of such side-shooter type head.
In the field of such side-shooter type recording head, the Japanese Patent Laid-Open Application Nos. 4-10940, 4-10941 and 4-10942 disclose a configuration in which a bubble generated by the heat from a heat generating resistor communicates with the external air to discharge the ink droplet (cf. FIGS. 3A to 3C).
Such configuration of the ink jet head allows a reduction in the distance between the ink discharge energy generating element and the orifice, in contrast to the conventional producing method for the side-shooter type ink jet head (for example as disclosed in the Japanese Patent Laid-Open Application No. 62-234941), and also to easily achieve recording with smaller ink droplets, thereby responding to the recent requirement for high-precision recording.
However, in the ink jet recording head produced according to the method described in the above-mentioned patent applications, as shown in FIGS. 3A to 3C, the thickness of the orifice plate remains substantially constant from above the ink supply aperture to the area of the bubble generating chamber and is very small because of the short distance from the ink discharge energy generating element to the orifice surface.
Such thin wall member, extended over a wide area and present in a suspended state, is extremely disadvantageous in terms of the strength of the recording head.
For example, such orifice plate may be broken by the paper jammed in the course of the recording operation, and may also become unreliable for the wiping operation with the wiping blade for the recovery of the ink discharge failure.
Also in case the orifice plate itself is composed of a resinous material, it may be swelled by the ink liquid, thus detrimentally affecting the ink discharge characteristics.
The thickness of the orifice plate may be increased for the purpose of increasing the strength thereof, but such increased thickness prolong the distance between the ink discharge energy generating element and the orifice surface, whereby it becomes extremely difficult to stably achieve the recording with smaller ink droplet, which is a strong means for high-precision recording.
Also in the configuration disclosed in the above-mentioned patent applications, it is found that the influence on the ink discharge characteristics and the recorded image, by the retentive bubbles resulting from the air dissolved in the ink becomes more conspicuous because of the smaller height of the ink flow path. In the following, such influence on the ink discharge characteristics and the recorded image, by the retentive bubbles resulting from the air dissolved in the ink, will be explained in more details. Usually, air is dissolved in the saturated state in the ink contained in the ink jet recording head. When the electrothermal converting element is activated in this state, in the course of repetition of the bubble generation by the phase change of the ink and the rapid diabetic contraction of the bubble, the air which has been dissolved in the ink may suddenly appear as a bubble of 1 .mu.m or smaller in diameter in the ink. Such bubble is known to dissolve again into the ink after a time determined by the bubble diameter, the surface tension of the ink, the saturated vapor pressure of the air etc. For example, a bubble of 1 .mu.s or less in diameter re-dissolves in the ink within a time of 1 .mu.s or less. However, in case plural electrothermal converting elements are activated in succession at a high frequency, such bubbles appear in plurality in the ink and mutually merge and grow before re-dissolution. It is also known that the time required for re-dissolution becomes significantly longer with the increase of the diameter of the bubble. As a result, plural retentive bubbles in the size of several tens to several hundreds microns are eventually stored in the ink jet recording head. Such retentive bubbles are scarcely re-dissolved in the ink and detrimentally affect the ink droplet discharge characteristics. More specifically, if such retentive bubble blocks the ink flow path, the nozzle cannot be filled with the sufficient amount of ink and results in defective ink discharge. Also if a giant retentive bubble (in the order of several hundred microns) is generated in the ink jet recording head and comes eventually into communication with the external air, the air may enter the nozzle and disrupt the ink meniscus, whereby the ink in the ink jet recording head is sucked into the ink tank by the negative pressure thereof and all the nozzles may become incapable of ink discharge. The most effective method for avoiding such detrimental effect of the retentive bubbles is the suction (or pressurized) recovery process of discharging the ink, with such retentive bubbles therein, from the discharge openings by suction or by pressurizing, before such bubbles grow to a size causing the detrimental effects. However, such method not only significantly increases the consumption of the ink but also deteriorates the throughput if such operation is conducted in the course of the recording operation. Another method is to eliminate such dissolved air from the air by a suitable method (degassing) and to use such degassed ink in the ink jet recording head. However, such method is only applicable to a large-scale printing apparatus, since such method is effective only for about several ten minutes after degassing and the device required for ink degassing is relatively large.